The present invention generally relates to power supply noise eliminating methods and semiconductor devices, and more particularly to a power supply noise eliminating method for eliminating power supply noise of a semiconductor integrated circuit which carries out a high-speed operation, and to a semiconductor device which uses such a power supply noise eliminating method.
In this specification, the "power supply noise" includes ground noise.
Semiconductor integrated circuits are used in systems with respect to which there are demands to realize high-speed operation and high performance. For this reason, there are demands to further increase the operation speed and to increase the number of terminals of the semiconductor integrated circuits. However, in order to cope with such demands requiring high-speed operation and large number of terminals of the semiconductor integrated circuit, a signal varying speed dI/dt and a number N of varying signals both become large, and power supply noise which is proportional to N.multidot.dI/dt is generated at a power supply or ground. This power supply noise is also proportional to an inductance of the terminals connecting the inside and the outside of the semiconductor integrated circuit. Accordingly, in general, the length of the terminals is made short and the terminals are coupled in parallel, so as to reduce the power supply noise by reducing the inductance of the terminals.
The number N of varying signals tends to increase considerably as the bus width of the system using the semiconductor integrated circuit increases from 8 bits to 16 bits, from 16 bits to 32 bits, and from 32 bits to 64 bits, for example. In addition, the signal varying speed dI/dt also increases considerably from several ns to 1 ns or less. For this reason, the value of N.multidot.dI/dt has become several tens times larger in recent years, and the power supply noise had a tendency of increasing therewith.
Power supply terminals, including ground terminals, of the semiconductor integrated circuit have an inductance on the order of several nH, and the power supply noise is generated by a rapid change in the current. In order to reduce this power supply noise, n power supply terminals are provided, where n is an integer greater than 1, so as to reduce the power supply noise to 1/n.
FIG. 1 is a circuit diagram showing an equivalent circuit of a power supply part of the semiconductor integrated circuit. As may be seen from FIG. 1, when n terminals are provided, an inductance L is reduced to 1/n or, a current deviation dI/dt of a current .sub..DELTA. i is reduced to 1/n, thereby reducing a voltage Vn=L.multidot.dI/dt which corresponds to the power supply noise to 1/n. In FIG. 1, Vcc denotes a power supply voltage.
However, an equivalent circuit of the actual semiconductor integrated circuit having a semiconductor chip is as shown in FIG. 2. In FIG. 2, R denotes a resistance with respect to a D.C. current which flows regularly, and C denotes a static capacitance of the semiconductor chip viewed from the power supply terminal. Accordingly, the equivalent circuit shown in FIG. 2 has a construction shown in FIG. 3 when viewed from a current source of the current .sub..DELTA. i.
An impedance Z of a parallel resonant circuit made up of the inductance L, the resistance R and the static capacitance C shown in FIG. 3 can be described by the following formula. EQU Z=x+jy EQU =1/{1/j.omega.L)+(1/R)+j.omega.C}
The following formula can be obtained by eliminating .omega. from the above formula. EQU {x-(1/2)R}.sup.2 +y.sup.2 +{(1/2)R)}.sup.2
When this formula is illustrated, a complex plane representation shown in FIG. 4 is obtained. In FIG. 4, the ordinate indicates the imaginary number, and the abscissa indicates the real number. For example, the resistance R is 50 .OMEGA., the inductance L is 2 nH, and the static capacitance C is 100 pF.
When FIG. 4 is illustrated in a .omega.-plane, a .omega.-plane representation shown in FIG. 5 is obtained. In FIG. 5, the ordinate indicates the amplitude, and the abscissa indicates the frequency. In addition, in FIG. 5, a solid line indicates the real number portion, and a dotted line indicates the imaginary number portion. A parallel resonance of the parallel resonant circuit occurs at a frequency .omega.p=1/.sqroot.LC, and an impedance corresponding to the D.C. current can be seen when viewed from the current varying point. As may be seen from FIG. 5, the impedance sharply assumes a large value at the parallel resonance frequency .omega.p. In this example, the impedance has a peak, that is, the parallel resonance point occurs, at a position where the impedance is approximately 50 .OMEGA..
Accordingly, the current deviation dI/dt as it is becomes the power supply voltage deviation. For example, if the current deviation dI/dt is 5 mA when the D.C. power supply current .sub..DELTA. i is 50 mA, a power supply voltage deviation Vn which is 1/10 the power supply voltage Vcc occurs at the parallel resonance frequency .omega.p of the parallel resonant circuit.
Conventionally, the power supply noise which occurs at the power supply or the ground and is proportional to N.multidot.dI/dt was reduced by reducing the inductance of the terminals connecting the inside and the outside of the semiconductor integrated circuit. However, a certain distance is inevitably required to connect the inside and the outside of the semiconductor integrated circuit, and there was a limit to increasing the number of terminals. For this reason, the inductance of the terminals does not become zero even if the length of the terminals is reduced within the possible range and the number of terminals is increased within the possible range, and there was a problem in that it was impossible to considerably improve the effect of reducing the power supply noise.
On the other hand, when viewed from the power supply terminals, the semiconductor integrated circuit has a static capacitance, and a parallel resonant circuit is formed by this static capacitance and the inductance of the power supply terminals when the current varies. As a result, the impedance becomes infinitely large at the parallel resonance frequency of the parallel resonant circuit, and there was a problem in that ringing noise is generated thereby. In addition, even if the inductance of the terminals is reduced, this would only increase the parallel resonance frequency of the parallel resonant circuit, and the parallel resonance point will not be eliminated. Hence, reducing the inductance of the terminals will not solve the problem of the ringing noise.